Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Available-for-sale
Amortized Cost	$ 4,971,510	$ 5,283,253
Gross Unrealized Gains	668	841
Gross Unrealized Losses	(472,637)	(498,375)
Fair Value	4,499,541	4,785,719
Held-to-maturity
Amortized Cost	3,400,403	3,502,411
Gross Unrealized Gains	1,176	2,843
Gross Unrealized Losses	(354,510)	(325,060)
Fair Value	3,047,069	3,180,194
Total debt securities
Amortized Cost	8,371,913	8,785,664
Gross Unrealized Gains	1,844	3,684
Gross Unrealized Losses	(827,147)	(823,435)
Fair Value	7,546,610	7,965,913
U.S. government and federal agency
Available-for-sale
Amortized Cost	484,694	485,005
Gross Unrealized Gains	16	11
Gross Unrealized Losses	(27,202)	(29,669)
Fair Value	457,508	455,347
Held-to-maturity
Amortized Cost	856,307	853,273
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(69,035)	(65,472)
Fair Value	787,272	787,801
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	322,921	321,993
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(21,338)	(22,774)
Fair Value	301,583	299,219
State and local governments
Available-for-sale
Amortized Cost	99,400	101,903
Gross Unrealized Gains	208	302
Gross Unrealized Losses	(3,969)	(3,273)
Fair Value	95,639	98,932
Held-to-maturity
Amortized Cost	1,636,726	1,650,000
Gross Unrealized Gains	1,176	2,843
Gross Unrealized Losses	(206,395)	(181,192)
Fair Value	1,431,507	1,471,651
Corporate bonds
Available-for-sale
Amortized Cost	14,860	27,007
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(620)	(756)
Fair Value	14,240	26,253
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	2,894,949	3,166,589
Gross Unrealized Gains	2	7
Gross Unrealized Losses	(336,931)	(355,333)
Fair Value	2,558,020	2,811,263
Held-to-maturity
Amortized Cost	907,370	999,138
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(79,080)	(78,396)
Fair Value	828,290	920,742
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,154,686	1,180,756
Gross Unrealized Gains	442	519
Gross Unrealized Losses	(82,577)	(86,570)
Fair Value	$ 1,072,551	$ 1,094,705